Catchpole, C. K., & Slater, P. J. B. (1995). Bird Song: Biological Themes and Variations. Cambridge: Cambridge University Press.

Studies in shelter medicine show that dogs with imbalanced gut bacteria exhibit more reactive, fearful, and impulsive behaviors. Veterinarians are now prescribing probiotics not just for diarrhea, but as an adjunct therapy for anxiety. The vagus nerve connects the enteric nervous system (the "second brain" in the gut) directly to the amygdala (the fear center). By feeding the microbiome, we are literally calming the animal's brain.

To make your paper academically robust, integrate these core animal science subjects:

Griffin, D. R. (2001). Animal Minds: Beyond Cognition to Consciousness. Chicago: University of Chicago Press.

In traditional veterinary curricula, students were taught to measure vital signs: temperature, pulse, and respiration. Yet, for the vast majority of animals, the most critical "vital sign" is their behavioral state.

: Behaviors modified by experience, including conditioning (e.g., a dog sitting for a treat), habituation, and social learning. Veterinary Behavioral Medicine